A whole process of a machining step for a machine tool consists of multiple steps, from a feeding starting moment, to idling process, to cutting starting moment, to cutting process, to cutting ending moment, to idling process and then to the feeding ending moment. Wherein, a cutting process refers to a process that when a cutting tool of the machine tool contacts with a workpiece and removes material. Online automatic judgment in the cutting process is a core for acquisition of key running parameters of the machine tool such as effective running time (cutting time) and the effective utilization (cutting time/running time). Besides, wear of the cutting tool, energy consumption for cutting, consumption of cooling fluid are all exist in the cutting process. Therefore, judgment about key moments in the whole process of the machining step plays an important role in contributing to real-time acquisition of service time of the cutting tool, online monitoring of effective energy utilization and online monitoring of the production status of the machine tool and the like.
At present, an existing cutting time acquired by communication of the computer numerical control machine tools is actually a tool feeding time of the machine tool, also called cutting feed time. However, the tool feeding time of the machine tool includes the cutting time and idling process time. As the service time of the machine tool increases, the idling process time also increases, so a cutting time error, generated by this method, of the machine tool will also increase.
Researches on cutting processes of machine tools are made throughout the world. According to Chinese Patent No. CN103838181A, a working process of a machine tool is judged according to a change characteristic of a current signal when a spindle of the machine tool is loaded. However, since current change characteristics in some finish machining processes are not so obvious and uncontrollable random fluctuations of current signals are relatively great, it is difficult to judge cutting starting and ending moments of such machining. According to US Patent (US 20140212236 A1), multi-element sensors are mounted at a cutting tool and a spindle to judge a cutting process of the computer numerical control (CNC) machine tools. However, mounting the sensor at the spindle of the machine tool may cost too much and even may influence normal work of the machine tool. In Canadian Patent (CA 2063887), a multi-displacement monitoring device is adopted to monitor a path of a cutting tool in a cutting process of a machine tool, but the accurate moment when the cutting tool contacts with a workpiece cannot be directly judged. In Chinese Patent No. CN104808584A, a device utilization state of a machine tool is monitored on line by use of a power change characteristic of the machine tool and a man-machine interaction method. However, cutting fluid and transparency of a safety door may influence accuracy of manual visual judgment, which reduces monitoring accuracy.
From the above, existing technologies for judging a cutting process for a machine tool mainly have the problems of high apparatus cost, inconvenience for mounting, lower judgment accuracy of some finishing cutting processes, difficulty in judgment about idling process and the like.